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Proof at last: Moon was created in giant smashup

It's a big claim, but Washington University in St. Louis planetary scientist Frédéric Moynier says his group has discovered evidence that the Moon was born in a flaming blaze of glory when a body the size of Mars collided with the early Earth.
The evidence might not seem all that impressive to a nonscientist: a tiny excess of a heavier variant of the element zinc in Moon rocks. But the enrichment probably arose because heavier zinc atoms condensed out of the roiling cloud of vaporized rock created by a catastrophic collision faster than lighter zinc atoms, and the remaining vapour escaped before it could condense.

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Moon formation: Was it a 'hit and run' accident?

Scientists have proposed a fresh idea in the long-running debate about how the Moon was formed.
What is certain is that some sort of impact from another body freed material from the young Earth and the resulting debris coalesced into today's Moon.
But the exact details of the impactor's size and speed have remained debatable.

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Title: A hit-and-run Giant Impact scenario
Authors: Andreas Reufer, Matthias M. M. Meier, Willy Benz, Rainer Wieler

The formation of the Moon from the debris of a slow and grazing giant impact of a Mars-sized impactor on the proto-Earth (Cameron & Ward 1976, Canup & Asphaug 2001) is widely accepted today. We present an alternative scenario with a hit-and-run collision (Asphaug 2010) with a fractionally increased impact velocity and a steeper impact angle.

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Title: A New Disintegrative Capture Theory for the Origin of the Moon
Authors: Peter D. Noerdlinger

The object that resulted in the creation of the Moon started in the same orbital path as Earth around the Sun, but at Earth's L4. This proto-Moon (PM) was 4 times less massive than the usual Giant Impact (GI) object "Theia" and was captured into Earth orbit. It had a 32% Iron-Nickel-Sulphur core supporting a dynamo, which explains magnetized lunar rocks. Following capture, it was torn apart by tidal forces and its core of iron plastered itself, with some of its rock mantle, on the surface of Earth at a very flat angle (producing the "Late Veneer"). After tidal stripping, the remaining PM rock was driven away from Earth to about 3.8 times Earth's radius and formed into what is now the Moon. The GI theory has several troubles: The violent collision melts the entire Earth, contrary to geological evidence. The Moon itself also has to condense out of the vapour cloud generated in the collision, but there is evidence that the Moon was not condensed out of vapour. In the new theory, the Moon as we know it may be only 3.8 - 3.9 billion years old, not 4.56 as usually assumed. That is the age of the PM. The minerals in the Moon would be about as old as the Earth, but would have been re-arranged in the capture and temporary disintegration process. If the Moon is as young as suggested, its origin would coincide with the beginning of life on Earth, which is unexplained in the GI theory. The manuscript asks, "Was the Moon Turned Inside-Out" and the answer is "Essentially, Yes."

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Moon Formation
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 Titanium signature poses puzzle for popular theory of Moon's origin.

A chemical analysis of lunar rocks may force scientists to revise the leading theory for the Moon's formation: that the satellite was born when a Mars-sized body smacked into the infant Earth some 4.5 billion years ago.
If that were the case, the Moon ought to bear the chemical signature of both Earth and its proposed 'second' parent. But a study published today in Nature Geoscience suggests that the Moon's isotopic composition reflects only Earth's contribution.

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Title: Equilibration in the Aftermath of the Lunar-Forming Giant Impact
Authors: Kaveh Pahlevan, David Stevenson

Simulations of the moon-forming impact suggest that most of the lunar material derives from the impactor rather than the Earth. Measurements of lunar samples, however, reveal an oxygen isotope composition that is indistinguishable from terrestrial samples, and clearly distinct from meteorites coming from Mars and Vesta. Here we explore the possibility that the silicate Earth and impactor were compositionally distinct with respect to oxygen isotopes, and that the terrestrial magma ocean and lunar-forming material underwent turbulent mixing and equilibration in the energetic aftermath of the giant impact. This mixing may arise in the molten disk epoch between the impact and lunar accretion, lasting perhaps 10²-10³ yr. The implications of this idea for the geochemistry of the Moon, the origin of water on Earth, and constraints on the giant impact are discussed.

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The Earth and Moon formed later than previously thought

The Earth and Moon were created as the result of a giant collision between two planets the size of Mars and Venus. Until now it was thought to have happened when the solar system was 30 million years old or approx. 4,537 million years ago. But new research from the Niels Bohr Institute shows that the Earth and Moon must have formed much later - perhaps up to 150 million years after the formation of the solar system. The research results have been published in the scientific journal, Earth and Planetary Science Letters.
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Title: Turbulent mixing of metal and silicate during planet accretion - And interpretation of the Hf-W chronometer
Authors: Tais W. Dahl and David J. Stevenson

In the current view of planet formation, the final assembly of the Earth involved giant collisions between proto-planets (> 1000 km radius), with the Moon formed as a result of one such impact. At this stage the colliding bodies had likely differentiated into a metallic core surrounded by a silicate mantle. During the Moon-forming impact, nearly all metal sank into the Earth's core. We investigate to what extent large self-gravitating iron cores can mix with surrounding silicate and how this influences the short-lived chronometer, Hf-W, used to infer the age of the Moon. We present fluid dynamical models of turbulent mixing in fully liquid systems, attempting to place constraints on the degree of mixing. Erosion of sinking cores driven by Rayleigh-Taylor instability does lead to intimate mixing and equilibration, but large blobs (> 10 km diameter) do not emulsify entirely. Emulsification is enhanced if most of the accreting metal cores deform into thin structures during descent through the Earth's mantle. Yet, only 1-20% of Earth's core would equilibrate with silicate during Earth's accretion. The initial speed of the impactor is of little importance. We proceed to evaluate the mixing potential for shear instabilities where silicate entrainment across vertical walls causes mixing. The turbulent structure indicates that vortices remain at the largest scale and do not mix to centimeter length scale, where diffusion operates and isotopes can equilibrate. Thus, incomplete emulsification and equilibration of accreting iron cores is likely to occur.
The extent of metal-silicate equilibration provides key information for interpretation of siderophile budgets and the timing of core formation using the Hf-W chronometer. The time scale of core formation derived from the Hf-W chronometer is usually tied to the last major metal-silicate re-equilibration, believed to coincide with time of the Moon-forming impact. However, we show that large cores have limited ability to reset the Hf-W system in the silicate Earth. Excess 182W in bulk silicate Earth is more sensitive to early core formation processes than to radiogenic ingrowth after the last giant impact.

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Master's thesis about core formation and the age of the Moon.

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The name of the hypothesized protoplanet is derived from the mythical Greek goddess Theia, a Titan who gave birth to the Moon goddess Selene. According to the giant impact hypothesis, Theia formed along with the other planets of our solar system about 4.6 billion years ago, and was approximately the size of Mars. One formation theory is that it would have materialized at the L4 or L5 Lagrangian points relative to Earth (in about the same orbit and about 60° ahead or behind), similar to a trojan asteroid.
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